ESA Science & Technology - No. 242 – End of Earth occultation season #12; start of Atmospheric Drag Experiment campaign #7

Cebreros ground station

Operations with the Cebreros (CEB) ground station were nominal.

There were data gaps in the science packets generated on 13 December for the MAG (magnetometer), and SPICAV (atmospheric spectrometer) and VIRTIS (imaging spectrometer) instruments. The cause is not known. However, a copy of the data is retained on-board the spacecraft until its receipt on the ground is verified. In this case, the data were retained on board until there was time in the schedule to downlink it to the ground again. The data were successfully downlinked and retrieved on 29 December.

Seventh Atmospheric Drag Experiment campaign

The seventh Atmospheric Drag Experiment (ADE) campaign started on 8 January 2012, as the pericentre altitude approached 175 kilometres above the surface of Venus. This is the latest campaign in a series that have been performed regularly since 2008. These campaigns provide measurements that are used to improve models of Venus's upper atmosphere, especially over its north pole.

The ESA ground station at New Norcia (NNO), Australia, and the NASA Deep Space Network (DSN) station in Canberra, Australia, provided tracking at pericentre during this period.

Venus Express's parking orbit is highly eccentric; it takes the spacecraft out to roughly 66 000 kilometres from the planet at its apocentre, and much closer to the planet's surface when the spacecraft is at pericentre.

For the ADE campaign, the entire spacecraft body and the attitude-sensing gyroscopes act as an instrument that detects the drag exerted by the atmosphere. By measuring its integrated effect on the spacecraft's attitude and position in orbit, the atmospheric density can be determined. During the campaign, the spacecraft's pericentre is allowed to decrease beyond its nominal altitude, down to 175 kilometres in this case (see figure) as the spacecraft flew over the northern pole of Venus. During closest approach, the spacecraft's solar panels are tilted to induce a very tiny roll in the spacecraft. This roll is countered by the reaction wheels, and the change in reaction wheel speeds can be used to determine the density of the atmosphere at this extreme upper altitude. The amount of drag experienced by the spacecraft is significantly lower than that which would be experienced during an aerobraking orbit change. The drag is maintained this level, which is known to cause no problems for the spacecraft.

The regular wheel off-loadings (WOL) that are used to release the accumulated spacecraft momentum every day were performed in the middle of the daily Cebreros communications passes. This provided at least one hour of tracking after the WOLs, allowing even minor orbit perturbations from the WOLs to be determined and removed from the orbit data, thus improving the accuracy of the ADE results.

Change in Venus Express's pericentre altitude with time. Credit: ESA

Occultation season #12

The twelfth Earth occultation season ended on 06 January 2012.

During occultation seasons, the geometry places Venus between the spacecraft and Earth for part of each orbit. This alignment allows the use of the spacecraft's Earth communications signal to probe the planet's atmosphere.

Shortly before Venus Express reaches pericentre, it is pointed to Earth and broadcasts its default signal. The signal used for occultation measurements must be far more accurate than required during a communications pass, hence it is generated using a special oscillator. As the spacecraft moves behind the planet, this highly accurate signal passes through the Venusian atmosphere before being blocked by the planet. The refracted signal can be measured and processed to yield details about the atmosphere that cannot be obtained in other ways. These occultation measurements are performed when Venus Express is at its pericentre, as and when the Earth-Venus-spacecraft geometry allows.

The ESA ground station at NNO was used as for occultation measurements, as the station faced Venus when the spacecraft was at pericentre.

The next occultation season beginning 27 March 2012 will be extremely unusual: the season's Earth-Venus-spacecraft geometry will last for about six months, as opposed to the typical two months for most occultation seasons.

Venus Express will fly over the terminator on 10 January, which happens roughly every four months. Contact with the Cebreros ground station will be skipped during this period, allowing the VMC (monitoring camera) to take extended images along the terminator in a thermally benign configuration.

Summary of main activities

The table below shows a chronology of the main spacecraft bus activities in the reporting period:

At the end of the reporting period on 14 January, Venus Express was 181.5 million kilometres from Earth; the one-way signal travel time was 605 seconds. The final oxidiser mass was 29.993 kg and the final fuel mass was 18.639 kg.

Payload activities

The instruments were operated nominally as per the plans of each instrument team.

Future milestones

• Orbital correction manoeuvres to raise the pericentre after the Atmospheric Drag Experiment.
• Start of the twentieth eclipse season on 8 February 2012. As the spacecraft passes into and then out of the shadow of Venus, the SPICAV and VIRTIS spectrometers can measure the changes in solar radiation as it passes through the different atmospheric layers.
• Quadrature operations: as the Earth-Venus-Sun angle exceeds 90 degrees, changes in antenna usage and restrictive thermal constraints become necessary. Quadrature season starts in March 2012.

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This report is based on four ESOC mission operations reports, MOR #317 through MOR #320. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.